2-methylpropene

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Non-GLP, non guideline study, published in peer reviewed literature, no restrictions, fully adequate for assessment

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

reference to same study

Cross-reference 2

Reason / purpose for cross-reference:

reference to other study

Data source

Reference

Reference Type:

publication

Title:

Disposition of inhaled isobutene in F344/N rats.

Author:

Henderson R, Sabourin P, Bechtold W, Steinberg B and Chang I

Year:

1993

Bibliographic source:

Toxicol. Appl. Pharmacol. 123:50-61

Materials and methods

Objective of study:

absorption

metabolism

Principles of method if other than guideline:

The purpose of this study was to determine the effect of exposure concentration on the uptake and metabolic disposition of inhaled isobutene.

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

isobutene

IUPAC Name:

isobutene

Constituent 2

Reference substance name:

2-methylpropene

EC Number:

204-066-3

EC Name:

2-methylpropene

Cas Number:

115-11-7

Molecular formula:

C4H8

IUPAC Name:

2-methylprop-1-ene

Details on test material:

Isobutene (99 + %) was obtained from Aldrich Chemical Co. (Milwaukee, WI, USA). [2-14C]isobutene (radiochemical purity of 98.5%, 2.7 mCi/mmol Lot No. 2484) was obtained from DuPont NEN Research Products (Boston, MA, USA). The purity of the [14C]isobutene was determined before use by HPLC

Radiolabelling:

yes

Test animals

Species:

rat

Strain:

Fischer 344

Sex:

male

Details on test animals or test system and environmental conditions:

Male F344/N rats (11-15 weeks of age) from the barrier-maintained colony of the Inhalation Toxicology Research Institute (ITRI) were used. The rats were housed in polycarbonate cages (two per cage) containing hardwood chip bedding and filter caps prior to the exposures. Rooms were on a 12-hr light-dark cycle with light starting at 6:00 AM. Food (Certified Lab Blox, Allied Mills, Chicago, IL, USA) and water were provided ad libitum.
Rats were preconditioned to the nose-only exposure tubes prior to start of the exposure. The rats were placed in the tubes for I hr and 3 hr, 2 days and I day prior to exposure, respectively. All rats were killed by ip injection of 250 mg sodium pentobarbital/kg body wt.

Administration / exposure

Route of administration:

inhalation: gas

Vehicle:

other: air

Details on exposure:

TYPE OF INHALATION EXPOSURE: nose only


GENERATION OF TEST ATMOSPHERE / CHAMPER DESCRIPTION
- Exposure apparatus: nose only inhalation chambers

- Source and rate of air: filtered room air

Duration and frequency of treatment / exposure:

2 hours

Doses / concentrations

Remarks:

Doses / Concentrations:
Target concentrations were 2, 40, 400, and 4000 ppm, ( 3.1, 75, 747, and 7474 mg/m3)

No. of animals per sex per dose / concentration:

Unlabelled isobutene: 3 rats per timepoint
Labelled isobutene: 5 rats per concentration, 3 rats per timepoint

Control animals:

no

Positive control reference chemical:

none

Details on study design:

Studies with unlabeled Isobutene:

Blood levels of isobutene and volatile metabolites. Male F344/N rats (11- 15 weeks of age) were exposed for 2 hr to 40, 400, or 4000 ppm isobutene in a nose-only exposure system. Blood levels of isobutene and volatile metabolites were determined by headspace analysis at 0.25, 0.5, 1.0, 2.0,2.25,2.75, 3.75, and 6.75 hr after initiation of the exposure.

Studies with [14C]Isobutene

Total uptake, excretion, and metabolism studies.
Male F344/N rats (11-15 weeks of age) were exposed to 40,400, or 4000 ppm [14C] isobutene for 6 hr in a nose-only exposure system. Five rats were exposed in plethysmograph tubes that allowed measurement of breathing patterns and the total amount of [14C]isobutene inhaled. These animals were killed and counted for 14C at the end of the exposure to determine total retained isobutene and the fraction of inhaled isobutene that was retained. Four exposed animals were placed in glass metabolism cages for the collection of excreta (urine, faeces, exhaled air) for 3 days after the exposure. After 3 days the amount of 14C in the excreta plus that remaining in the carcass were summed to determine if a mass balance was achieved with the original total body burdens determined in the rats killed immediately after the exposure.

In rats exposed to 40 ppm [14C]isobutene, blood samples were collected in separate rats (three per group) at 0.5, 1,2,4,6,6.5,7.5,9.5, 15, and 24 hr after initiation of the exposure. Blood and excreta samples were analyzed for metabolites..
Because results indicated nonlinear uptake and metabolism of isobutene over the exposure range of 40-4000 ppm [14C]isobutene for 6 hr, an additional group of rats was exposed to 2 ppm isobutene
for approximately 3 hr. The purpose of this exposure was to define a linear range of exposures for uptake and metabolism of isobutene.

Details on dosing and sampling:

see above

Statistics:

none

Results and discussion

Preliminary studies:

none

Toxicokinetic / pharmacokinetic studies

Details on absorption:

The blood concentrations of isobutene at the end of the exposures (at equilibrium) were 7.7, 90.4, and 1552 ng/mL blood for the 39, 434, and 3790 ppm exposures, respectively. The ratio of blood concentrations (in ng/mL) to exposure concentrations (in ppm) was 0.20, 0.21, and 0.41 for
the three exposure levels, indicating that the blood concentration of isobutene increased more than proportionately at the highest exposure concentration. This indicates that the capacity of the rats to metabolise isobutene was exceeded at the higher exposure concentrations resulting in higher blood levels.

Details on excretion:

The percentages of the initial body burden excreted by different routes or retained in the body did not differ statistically for the three lower exposure levels, but the highest exposure level resulted in a greater fraction of the isobutene being exhaled. This demonstrated that at 4000 ppm, the ability of the rats to metabolise isobutane is exceeded. This results in a higher percentage of isobutene being excreted in exhaled air.

Toxicokinetic parameters

Toxicokinetic parameters:

other: not determined

Metabolite characterisation studies

Metabolites identified:

yes

Details on metabolites:

Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol.

Any other information on results incl. tables

Fate of [14C]Isobutene at End of Exposure (as % of Total Body Burden mean ± SE, n = 4) 

	Exposure concentration of isobutene
	2.12 ppm	44 ppm	417 ppm	3930 ppm
Faeces	80.0 ± 0.6	88.4± 0.3	82.4± 0.9	62.4± 2.8
CO2	0.87 ± 0.29	1.33± 0.14	1.81± 0.14	0.64± 0.16
Exhaled isobutene	7.6 ± 0.2	4.6± 0.1	5.1± 0.2	13.7± 0.9
Carcass	6.4 ± 0.5	1.8± 0.2	7.2± 0.4	20.7± 2.3
Urine	5.1 ± 0.2	3.9± 0.4	4.6± 0.6	2.5± 0.9

Applicant's summary and conclusion

Conclusions:

Interpretation of results (migrated information): low bioaccumulation potential based on study results
Saturation of the metabolism of isobutene (2-methylpropene) occurred in rats. Oxidised metabolites were excreted in the urine.

Executive summary:

 Male F344/N rats were exposed to isobutene (2-methylpropene) by inhalation at concentrations from 40 to 4000 ppm (91.8 to 918 mg/m³). A time-course evaluation of blood levels of isobutene was performed and urinary metabolites were determined. Blood levels of isobutene were linearly related to exposure up to 400ppm (918 mg/m3) but were supralinear at 4000 ppm (9180 mg/m3) indicating saturation of metabolism at this higher dose level. Absorption of inhaled isobutene was about 8% up to 40ppm but decreased at higher concentrations. At 40 ppm, over 90% of absorbed isobutene was metabolised but at 4000 ppm 20% was excreted as unchanged isobutene also indicating saturation of metabolism. Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol.